Mammalian brain microtubules, polymerized in vitro, and sea urchin sperm outer fibers, fractionated to A-singlet microtubules, will be studied by x-ray diffraction of oriented wet gels and by optical diffraction of negatively-stained images. The basic monomer surface lattices and the positioning of specific tubulin dimers within the lattice will be determined. Through isomorphous replacement, attempts will be made to determine the shapes of monomeric polypeptide chains at a moderate resolution x-ray level. Image reconstruction techniques will also be used to determine general features of monomer and dimer conformation.